We propose to test the hypothesis that lean body mass (LBM), here defined as body weight minus ether-extractable fat, is largely determined by genetic factors. To this purpose a series of monozygous (MZ) and dizygous (DZ) like-sexed twins will be compared with respect to estimates of LBM by K40 counting. Our own work has shown that for subjects of a given age and sex, LBM has a smaller coefficient of variation than weight, and that the frequency distribution of LBM has much less skewness than that of body weight. LBM comprises most of the metabolically active tissues of the body, and thus may be a basic summary measurement of the result (phenotype) of interaction among the genes controllng growth and development. In addition to determining the heritability of LBM, we will, in the process of genotyping for twin zygosity, begin to accumulate data suitable for a preliminary evaluation of the relationship between LBM and the extent of sharing specific chromosome haplotypes within like-sexed, dizygous twin pairs.